3. OBSERVATIONS

To understand the nature of the BBB we first need to determine its SED
and thus determine its strength compared to the AGN's SED. Although this
task seems relatively simple, there are a lot of observational
problems. One of the most important problems is that a substantial part
of the BBB in any particular source is in the unobservable part of the
electromagnetic spectrum (13.6 eV - 0.1 keV), where it is hidden by
Galactic absorption. Therefore, the characteristics of this region need
to be inferred from theoretical considerations. In the optical/UV
wavelength region, where AGNs can be observed, the spectra have the
characteristic strong broad emission lines whose wings make assignment
of "line-free continuum" regions very difficult. Further, the Balmer
continuum and the many blended Fe II emission
lines produce a relatively smooth feature in the AGN spectrum that also
has to be accounted for in the continuum fitting process
(Wills, Netzer, & Wills
1985).
Thus, in this section we discuss the shape of the BBB by considering the
optical/UV region and the far-UV/soft X-ray regions separately. We also
discuss in detail the observational caveats in each spectral region. We
will often discuss composite spectra that have been generated by
co-adding several hundred optical/UV spectra in the AGN rest
frame. These composites serve as a high signal-to-noise ratio reference
to the "observed" AGN shape, because although individual AGN spectra
differ in detail, they all have similar characteristic features. It is
important to remember, however, that a composite spectrum really has
no physical meaning, and models must be fitted to spectra of
individual sources.

The median optical to UV continuum slope in radio-quiet QSOs is
-0.32
(F;
Francis et al. 1991).
The slope was determined in the 1300-5500 Å region after the Balmer
continuum and the Fe II line emission were
accounted for in the continuum fitting process. Using International
Ultraviolet Explorer (IUE) data,
O'Brien, Gondhalekar,
& Wilson (1988)
found that the rest-frame UV slope (1200-1900 Å) is slightly
steeper than the optical and that higher luminosity objects have flatter
slopes than lower luminosity objects. The rest-frame UV slope (1050-2200
Å) from HST data is -0.99
(Zheng et al. 1997).
It must be noted here that there is substantial scatter in the slopes of
the individual objects, yet the optical/UV continuum is redder than the
long wavelength prediction of stationary accretion disk models.

The two recent HST and Roentgen Satellite (ROSAT)
investigations by
Zheng et al. (1997) and
Laor et al. (1997),
respectively, allow us to investigate observationally the "observed"
shape of the BBB in the 13.6 eV-1 keV region of the continuum. The UV
composite spectrum of Zheng et al. was compiled using HST Faint
Object Spectrograph (FOS) observations. This sample contained quasars
with mean redshifts ~ 1, which allowed determination of the UV composite
spectrum in the rest wavelength range ~ 300-3000 Å (see
Fig. 2). The individual spectra in the composite
have been corrected for Galactic reddening and intergalactic
absorption. The UV composite spectrum of AGNs does not appear to rise to
shorter wavelengths but seems to roll over shortward of 1000 Å.
Laor et al. (1997)
produced a composite soft X-ray spectrum of a complete sample of low
redshift quasars using ROSAT Position Sensitive Proportional
Counter (PSPC) observations. The soft X-ray data seem to be just an
extension of the UV composite spectrum (see
Fig. 3). If taken at face value, the UV and X-ray
composite spectra indicate that the BBB may not be as energetically
dominant as thought previously. Further, the peak of the "observed" BBB
does not seem to be in the extreme-UV (100-800 Å) but rather in the
far-UV region (900-1100 Å).

Figure 2. UV composite spectrum compiled
using HST / FOS observations. The individual spectra in the
composite have been corrected for Galactic reddening and intergalactic
absorption. Note that the composite spectrum does not appear to rise to
shorter wavelengths but seems to roll over shortward of 1000
Å. The 1050-2200 Å region can be fitted by a power-law
continuum
F-0.99. Shortward
of 1000 Å, the spectrum in
F is close
to -2 and seems to
allow a smooth extension into the soft X-rays (courtesy W. Zheng).

Figure 3. Composite "observed" optical-soft
X-ray spectrum for AGNs. The composite UV spectra for radio-loud (RL)
and radio-quiet (RQ) objects are plotted as a thin solid line. The
thick solid lines represent the optical and soft X-ray composite
spectra. The dotted line is the spectral shape predicted by
Mathews & Ferland
(1987),
while the dashed line shows a simple power-law model (not an
accretion disk) with a thermal cutoff corresponding to T = 5.4
× 105 K. The data suggest that the far-UV power-law
extends into the soft X-ray regime. The 0.2-2.0 keV spectral slope is
much flatter than predicted by the simplest thin accretion disk models,
which have exponential cutoffs in this wave band (courtesy
A. Laor).

There are a number of observational issues that can affect the observed
shape of the BBB. These issues should especially be kept in mind when
considering the shape of the BBB, as determined by the UV and X-ray
composite spectra, because they affect each individual object in a
different manner and thus possibly create an unphysical effect in the
composite spectrum. Below we discuss the various issues in detail.

Host galaxy contamination. - The optical/UV region of an AGN
spectrum is also where stars put out most of their radiation. Thus
careful galaxy subtraction is necessary if we are to determine the shape
of the BBB accurately. In low-redshift AGNs, such corrections are often
made by subtracting with galaxy bulge template spectra. However, this
may be inadequate because the correction does not effectively correct
for a starburst population. HST imaging observations of Seyfert
galaxies show that nuclear star formation is often occurring well within
0".5 - 1".0 of the nucleus. Depending on the dominance of
these star-forming regions, they can contribute substantially toward the
optical/UV light that is seen in most ground-based apertures. A
comparison of contemporaneous HST and ground-based spectroscopy
of NGC 7469 clearly shows this problem (see
Figs. 4 and 5). In this
particular object, ~ 70% of the starlight falls within 0".1 of the
nucleus! For ground-based observations, this problem is exacerbated
owing to effects of seeing. Although nuclear starbursts are likely to be
present in the host galaxies of high-luminosity AGNs (QSOs), more than
80% of the radiation is from the QSO. Therefore for these objects we do
not expect substantial contamination in the optical/UV region from the
host galaxy.

Figure 4. The HST Wide Field
Planetary Camera 2 image of the nuclear region of NGC 7469. The
3" diameter starburst ring is clearly visible. The nucleus of the
galaxy is saturated in this image. Note that in a ground-based slit
spectrum, depending on the slit (usually 1" in width) orientation,
the ring contaminates the spectrum. Further, such rings would also have
contaminated the UV spectra obtained by IUE whose aperture was
10" × 20". The square represents the 0".86 square
FOS aperture (courtesy W. Welsh).

Figure 5. Comparison of the contemporaneous
HST / FOS (lower curve) and ground-based spectroscopy of
NGC 7469. The HST / FOS spectrum is through a
0".86 square
aperture, while the ground-based spectroscopy is mostly through a
10" × 16".8 slit (similar in size to the IUE
large-aperture data). The estimated host galaxy contamination is ~ 18%
at 5400 Å, which we see in the figure is highly
underestimated. The gross difference between the HST / FOS and
ground-based fluxes is due to the host galaxy and starburst ring
contribution in the ground-based spectrum. This figure highlights the
value of high spatial resolution spectroscopy (courtesy W. Welsh).

Intrinsic reddening. - Intrinsic reddening in the AGN plays an
important role when determining the optical/UV continuum. It is
generally assumed that the intrinsic reddening in AGNs is small
[E(B - V) = 0.05-0.1 mag], independent of the
reddening law. Although this small amount of reddening does not affect
the optical fluxes dramatically, it has a dramatic effect at UV
wavelengths. This reddening correction becomes extremely crucial for
high-z AGNs since the reddening correction is applied to the
far-UV rest wavelengths. Thus applying a reddening correction, even a
very small one, can effectively change the shape of the observed BBB
spectrum.

In most analyses the continuum is corrected for Galactic reddening, and
there may sometimes be an attempt to correct for intervening
intergalactic absorption. An attempt to correct for intrinsic reddening
via line ratios is also often made, but depending on the line ratios,
one can get significantly differing amounts for the correction. In
addition, the reddening might be different for the line- and
continuum-emitting regions. A systematic analysis of the reddening
indicators and effects of dust in multiwavelength spectra needs to be
undertaken, especially now that we have mounting evidence for dust in
the nuclei of AGNs
(Malkan, Gorjian, &
Tam 1998).

The UV composite spectrum generated by
Zheng et al. (1997)
has a spectral break at 1000 Å. Could this prominent continuum
feature be due to inaccurate corrections for intrinsic reddening in each
individual AGN?
Laor & Draine
(1993)
have shown that dust opacity peaks at ~ 700-800 Å and drops sharply
at shorter wavelengths. If the far-UV spectral slope change in the Zheng
et al. spectrum was due to dust, then below 700 Å the composite
spectrum should have "recovered" by 400 Å. Such a change in
continuum slope is not seen, which indicates perhaps that the effects of
intrinsic dust reddening are not important. However, it must be noted
that the far-UV spectral region of the composite has few objects
contributing to the spectrum. These objects are at high redshifts, and
their continua have been statistically corrected for intervening
intergalactic absorption (for clouds with NH <
1016 cm-2), and additional corrections if the
objects were known to have intervening absorption systems.

In an individual spectrum, reddening will not cause a spectral break at
1000 Å but will just change the slope of the spectrum dramatically
at these short wavelengths. In fact, because of the composite nature of
the spectrum, it may be possible that the spectral break is prominent
owing to incorrect reddening corrections in each of the individual
spectra that extend over limited regions of the entire wavelength range
of the composite. Notwithstanding the above observational caveats, the
incidence of the spectral break at 1000 Å, so close to the Lyman
limit, is tantalizingly suggestive of an intrinsic emission mechanism as
the cause.

Extreme-UV energy distribution. - As mentioned earlier, to
understand the primary emission mechanism in AGNs, it is crucial to know
the shape of the BBB in the extreme-UV (EUV). We have to determine if
the UV composite truly represents the EUV SED in AGNs. At wavelengths
less than 800 Å, only a few objects are contributing toward the UV
composite of
Zheng et al. (1997).
There are very few observations of AGNs in this region that can be used
for a quantitative analysis of the EUV energy distribution. For both
Seyfert galaxies and QSOs, there are individual objects in which the BBB
SED is not similar to that derived from the UV and X-ray composite
spectra (e.g., HS 1700+6416, HS 1103+6416, and HE 2347-4342 from
Reimers et al. 1998;
PG 1211+14;
Zheng et al. 1999).
Figure 6 shows the far-UV and EUV SED in
luminous AGNs. The variety of the EUV SEDs indicate that describing the
BBB with simple models will not be possible and that the task is much
more complex, and the SED at these short wavelengths needs to be
quantified better.

Figure 6. Spectral energy distributions for
four high-redshift QSOs detected in the Hamburg quasar survey. The
spectra are shown in the rest frame of the quasar. The UV continua
derived for the dereddened spectra have been corrected for H I continuum absorption of the Lyman limit systems. An
additional correction for the cumulative H I
continuum absorption of
Ly clouds with log
N(H I)
16 cm-2 is
indicated by the dotted lines. We see that each of these AGNs show
dramatically different EUV distributions (courtesy D. Reimers).

Soft X-ray issues. - The soft X-ray excess component in AGNs is
important for the evaluation of accretion disk models because it is
often interpreted as the high-energy tail of the BBB and it helps
determine the total energy emitted in the BBB. There are a number of
spectral components that contribute to the X-ray excess region (0.1 to ~
1 keV), including the underlying X-ray power law and the "warm absorber"
(absorption in the 0.5-1.5 keV range dominated by O VII and O VIII K-shell
absorption edges in material with NH ~ 1022
cm-2). All these spectral components complicate the
theoretical interpretation of this region. A detailed analysis of
ASCA data by
George et al. (1998)
challenges the presence of the soft X-ray excess in some objects, and
recent BeppoSAX data fail to show a soft X-ray excess in many
sources in which such an excess was claimed to exist before
(Matt 1998).
The presence of a warm absorber in Seyfert galaxies and the lack of warm
absorbers in high-luminosity AGNs has lead to some extent to more
observational uncertainty.

To add to the modeling difficulties, simultaneous data obtained with
ASCA and ROSAT showed different slopes in the 0.2-1 keV
bands. The soft X-ray slopes observed by
Laor et al. (1997)
need to be verified because there have been observational issues
concerning the accuracy of the ROSAT PSPC calibration matrix,
especially at low flux levels (which is obviously the case for X-ray
observations of AGNs). However, the H I columns
determined by Laor et al. agree remarkably well with the accurate 21 cm
H I columns, which suggests that the ROSAT
PSPC calibration matrix may be quiet reliable.

It is difficult to compare the ROSAT and ASCA
observatories. ASCA cannot observe the soft X-rays
( 0.6 keV) and has
a broad wave band (0.4-10 keV) and high energy resolution, while the
ROSAT PSPC observed soft energies (0.1 keV) but did not have a
broad wave band or high energy resolution. The presence of numerous
spectral components and their relative contributions at various energy
bands further complicates comparisons between the two
observatories. Thus, interpretation of the soft X-ray excess depends on
our understanding of the broader X-ray wave band.

Sample consistency. - The UV X-ray composite shown in
Figure 3 is based on two different samples. The
far-UV composite is based mainly on quasars with z > 1, while
the soft X-ray composite is based exclusively on quasars with z
< 0.4. Thus, if the quasar SED evolves with z, it may be
misleading to combine the SED of high- and low-z samples. One
clearly needs to measure the far-UV and soft X-ray continuum for the
same population of quasars in order to establish the shape of the BBB.